Looking back now, it’s clear that Bernie Sanders reached the apex of his political power in the weeks leading up to the June 7 California primary. His improbable rise had galvanized millions. He’d put a scare into Hillary Clinton. Although he had no plausible shot by that point of winning the Democratic nomination, he looked capable of carrying California, which would have inflicted real damage on his rival. Clinton understood this and signaled she was ready to bargain for his endorsement. He never placed the call.

Sanders lost California, and he lost the nomination. And with each day that he withholds his endorsement, he loses a little more of the political capital he gained during the primaries. Sanders undoubtedly shifted the balance of power in the Democratic Party and exposed its generational future. But his personal involvement in these changes is quickly fading.

History is full of examples of candidates who ran thrilling primary races, came up short, and then translated that excitement into tangible gains: a key cabinet post (Clinton), a future candidacy (Ronald Reagan, Gary Hart), or influence in the next administration through personnel appointments or policy commitments.

Sanders could have ranked among them. But, for reasons rooted in his personality and aloof political style, it looks like he won’t. He’s trapped by an inability, baffling even to some of his supporters, to end his campaign on advantageous terms. For weeks he’s swerved like a loose fire hose between gruff suggestions of support for Clinton—saying he’ll do all he can to stop Donald Trump—and threats to keep fighting her straight through the convention, possibly expecting she’d be indicted for maintaining a private e-mail server as secretary of state. (On July 6, the day after the FBI said it wouldn’t recommend any charges, the Department of Justice said none would be pressed.)

Asked recently by NBC’s Andrea Mitchell why he wouldn’t back Clinton, Sanders replied as if the primary battle were still raging: “It’s not a question of my endorsement. It’s a question of the American people understanding that Secretary Clinton is prepared to stand with them as they work longer hours for low wages, as they cannot afford health care, as their kids can’t afford to go to college. Make it clear that she is on their side, that she is prepared to take on Wall Street, the drug companies, fossil fuel industry. Deal with the global crisis of climate change. I have no doubt that if Secretary Clinton makes that position, those positions clear, she will defeat Trump and defeat him by a very wide margin.” Here was Sanders still piously insisting on complete capitulation—even though Clinton beat him by a larger margin than Barack Obama had beaten her eight years earlier.

What’s the matter with Sanders? Every Democrat not fully in thrall to him wants to know. Clinton officials privately seethe at his continued criticism. Liberals who once cheered his ascent now worry he could divide the party with catastrophic results: a Trump presidency. Sympathetic voices such as Washington Post columnist E.J. Dionne have nervously reminded Sanders of his “moral obligation to help Clinton,” while the Huffington Post has veered toward outright alarm at his “bewildering” behavior.

In running for president, Sanders transformed Democratic politics by demonstrating that blunt talk about economic inequality can animate a broad portion of the electorate and isn’t the political killer Democratic strategists believed. For years, the Republican charge of “class warfare” cowed many Democrats from speaking out. Sanders showed they needn’t worry any longer. “The charge of ‘class warfare’ was a very powerful obstacle for us for years, and it died this year. I give Sanders some credit for that,” says Barney Frank, the liberal former congressman from Massachusetts and longtime Sanders colleague. “But I think he’s already eroded much of what he gained. If the goal is to maximize your impact on public policy, then he’s making serious mistakes. There are more and more people angry at him.” A Sanders spokesman declined to comment.

And Sanders isn’t just undermining Clinton. Whether or not he realizes it, he’s undermining his self-appointed role as Wall Street scourge and liberal standard-bearer, because he can’t figure out how to lose.

Two months ago he seemed destined to be a consequential figure who would guide the Democratic Party leftward for years to come. Now, with President Obama and Massachusetts Senator Elizabeth Warren stepping in to unify the party behind Clinton, Sanders increasingly looks like an afterthought who’s squandering an historic opportunity.

Despite the passions he aroused in the primaries, Sanders may be uniquely ill-equipped to carry off the transformation of Washington he’s long called for. The same qualities that his supporters find so appealing—his independence from any party establishment and his open disdain for the political process—are also reasons why he has so few legislative accomplishments, even after decades in Congress.

“Sanders’s approach for 25 years has been, ‘I will say what is the right thing to do, and then criticize anybody who doesn’t join me,’ ” Frank says. “It’s not simply a refusal to compromise. It’s a focus on being almost morally superior to everyone else. Eschewing compromise is part of that.”

Over that time, Sanders operated as a free rider, availing himself of all the benefits of party membership, even as he pointedly stood outside the party system by declaring himself an independent. “He benefited in the House and Senate from the Democratic majorities and the seniority system that let him move into [senior] positions without having to actually do anything to help other members,” says John Lawrence, former chief of staff to Democratic Minority Leader Nancy Pelosi. “In fact, he made a point of not doing that. He’s been a member of the club without having to pay dues.” As Sanders himself has admitted, he became a Democrat on the eve of the campaign purely for the sake of expediency: It was his best shot at the White House. It’s no accident that he drew fewer endorsements from his congressional colleagues than Ted Cruz.

Sanders has used his career in Congress to signal his own virtue relative to other politicians—also the message of his presidential campaign. “The lecture he delivers on the trail about how ‘millionaires and billionaires’ have a lock on the political process is the same one he gives to his colleagues behind closed doors,” says Jim Manley, former communications director for Senate Minority Leader Harry Reid. But politics is a grubby business, and purity comes at a price. Sanders’s contemporaries in Congress include major liberal legislators whose careers produced landmark reforms: Frank (the Dodd-Frank financial reforms) and former California Representatives George Miller (No Child Left Behind) and Henry Waxman (the Clean Air Act, the Affordable Care Act) among them. In their personal politics, each inhabited a space with Sanders along the left wing of the Democratic Party.

What distinguished Frank, Miller, and Waxman, however, was their willingness and ability to marry idealism with pragmatism, to claim partial victories that, over time, led to historic changes. They didn’t wait for a revolution. In fact, during Reagan’s presidency, as dark a period for liberalism as any in recent memory, Waxman struck a series of deals to expand Medicaid coverage—an oft-stated goal in the Sanders litany. And though all three legislators are now retired, their influence lives on in generations of former staffers who fanned out across Capitol Hill and into the Obama administration. Sanders’ most prominent ex-staffer, his campaign manager, Jeff Weaver, runs a comic book store in Virginia.

Although Sanders won 22 states and 12 million votes in the Democratic primaries, his orientation has never been toward the party. This is why he won’t end his campaign in accordance with the usual protocol. The enticements that typically persuade a runner-up to fall in line behind the nominee—loyalty to party and the chance to advance within its power structure—hold little appeal for someone determined to go it alone. “I think he views the excitement he built during the campaign as a product of support for him personally, rather than for the Democratic Party or Hillary Clinton,” says Lawrence, who spent years working for Miller before Pelosi. “He wants to build a movement tied to principle, not to party, which is how he has always functioned. That minimizes his need to ingratiate himself with party leaders or ask his supporters to accept a nominee they don’t like.”

But it presents Sanders with the difficult question of how to harness his newfound stature. The skill you need most when you’ve fallen just short of the nomination is the ability to negotiate concessions that advance your goals and extend your influence, even with someone else atop the ticket. Sanders has never shown any interest or facility in exerting power through the usual channels. He abhors the whole process. That’s why he’s stuck: If you reject the idea of compromise, it’s impossible to settle for anything less than outright victory.

If Sanders wants to shape Democratic politics, he’ll confront the timeless Washington dilemma of whether to maintain his ideological purity or sit down and bargain with the enemy. His lone legislative achievement of any significance, a veterans bill he passed with Arizona Senator John McCain, suggests he’s at least capable of the latter. When Sanders first introduced the bill, it went nowhere. Only after enlisting McCain, and making significant concessions, did the revised version pass into law.

Sanders does finally appear to be inching toward a unifying moment with Clinton. “That’s a process we’re working on that could lead to an endorsement before the convention,” says Sanders spokesman Michael Briggs. Still, Sanders seems determined to avoid making concessions to Clinton, even if she manages to wrench an endorsement from him. “His real objective is to keep together the movement he’s implausibly united,” Lawrence says. “But part of doing that is not becoming tainted by looking like a typical party figure who, having run and lost, just defers to the nominee.” So Sanders has begun engaging in some uncharacteristic behavior, penning a flurry of newspaper op-eds and starting a fight over the party platform—a sort of simulacrum of how he imagines establishment influence is applied.

These actions don’t require Sanders to compromise his beliefs, but neither are they likely to have much practical effect. The platform is a purely symbolic document with no hold over the policies pursued by the nominee. It’s usually ignored. (As Trump put it to Bloomberg Businessweek in May: “I don’t care who writes the platform. I have the loudspeaker.”) Although Sanders succeeded in pushing the Democratic platform to the left—the latest version includes a financial-transaction tax, decries “the greed and recklessness of Wall Street,” and blocks finance executives from serving on the boards of regional Federal Reserve banks—this is mostly a feel-good achievement with few real-world implications. “The platform is the ‘Miss Congeniality’ of the beauty pageant,” Frank says. “Do you remember any significant platform fights? No. Nobody does.”

Sanders vows to keep on fighting anyway. “This is a document that needs to be significantly improved,” he declared in a July 3 op-ed for the Philadelphia Inquirer. “There were a number of vitally important proposals brought forth by the delegates from our campaign that were not adopted.”

Keep up with the best of Bloomberg Politics.

Get our newsletter daily.

Business

Your guide to the most important business stories of the day, every day.

You will now receive the Business newsletter

Markets

The most important market news of the day. So you can sleep an extra five minutes.

You will now receive the Markets newsletter

Technology

Insights into what you'll be paying for, downloading and plugging in tomorrow and 10 years from now.

You will now receive the Technology newsletter

Pursuits

What to eat, drink, wear and drive – in real life and your dreams.

You will now receive the Pursuits newsletter

Game Plan

The school, work and life hacks you need to get ahead.

You will now receive the Game Plan newsletter

Many Sanders supporters cheer his fixity of purpose. That’s the essence of his approach to politics. Although with the course he’s chosen, he’ll probably end up achieving much less than he might have. When it became clear that Clinton would be the Democratic nominee, most people imagined that Sanders would return to the Senate greatly enhanced, where he and Warren—provided she doesn’t join the ticket—would form a powerful liberal vanguard. But this would require Sanders to abandon the role of gadfly, which he appears curiously unwilling to do. “The question has always been, will he change his tactics to be more influential in the Senate?” Manley says. “I see no indication that he will.”

He’s marching back to assume the same position he held before he ran for president: a one-man army in an institution that requires 60 votes—pure of heart, righteously angry, and almost entirely ineffective.